Wednesday, September 20, 2006

Rutherford’s Experiment

Lesson Title: Rutherford’s Experiment

Learning Outcome:
At the end of the sessions, you must be able to explain Rutherford’s experiment to prove the existence of the nucleus.

Learning Presentation:
Rutherford as you might remember contributed to the idea of what an atom might have looked like click here to find out how he did it

If you want additional references to help you know him better check out your textbook its on pages 61 and 62. :)

In 1899, J. J. Thomson proposed a model of the atom. The atom according to him is a positive ball of electricity wherein negatively charged particles are embedded. For this time, this so-called plum pudding model was accepted.

In 1908-1909, Ernest Rutherford and his co-workers performed the well-known gold foil experiment. They hammered a gold foil into a thin sheet that was a few atoms thick. They fired alpha particles at the thin gold foil. An alpha particle is positively charged (charged 2+) with a mass four times (mass 4 u) that of a H-atom.

Rutherford tested Thomson’s model of an atom. H e concluded that if his model were correct, most of the alpha particles would pass through a thin gold foil with very slight deflections, because the positive charge of the atom was diffused.

play this video to have a clearer and better understanding of the atom as proposed by Rutherford

Rutherford was surprised by the result of his experiments. His observations were the following:
Most of the alpha particles passed through the gold foil undeflected.

A few alpha particles passed through the gold foil with large angles of deflection.
Still fewer alpha particles bounced back in the direction from which they came.

The bouncing back and scattering of the alpha particles could not be satisfactorily explained using Thomson’s model of the atom.

After several years of study, Rutherford interpreted his observations. His conclusions were:
The atom has a central core called the nucleus. It is here where the mass of the atom is concentrated.

The charge of the nucleus is positive.

The nucleus is very small compared to the volume of the atom. The atom is mostly empty space.

The electrons are found outside the nucleus.

Later measurements showed that diameter of a nucleus are very small (about 10-12 cm) compared to the diameter of an atom (1 to 5 x 10-8 cm). This is about the size of a marble compared to the size of a softball field (atom).

To measure if you have gained sufficient knowledge about the atom try this activity
Atomic Hangman

How did he discovered the nucleus? how did the deflection of the alpha particles led him to say that there is a nucleus? Place your answer by clicking here

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Cathode Rays and the Electron

Lesson Title: Cathode Rays and the Electron

Learning Outcome:
At the end of the sessions, you must be able to:
1. Discuss what cathode ray and radioactivity reveal about the atom.

Learning Presentation:
An atom is so small and yet it still have sub particles, how can you discover something so small that it has never been seen????????????

How did his experiment lead him to discovery this unseen particles

What did he discovered? click here

If you want to use your check another reference and get to know this person and his discovery better, check out your textbook its on pages 59 and 60.

1. Much of our understanding of the structure of the atom came from the study of electric charges in rarified gases. William Crookes (1832-1919), an English chemist, using a special vacuum discharge tube later called Crookes’ tube, show that there was some kind of matter coming from the cathode (negative electrode).

2. These particles were called cathode rays because they originated from the cathode. Numerous experiments made by Julius Plucker, Johann Hittarf, William Crookes, and others showed that cathode rays exhibited the following properties:
a. Cathode rays are emitted from the cathode when electric current is passed through an evacuated tube containing a gas at very low pressure.
b. The rays travel in a straight line as indicated by the shadow cast by small objects placed along its path. Metal objects struck by the rays become red hot.
c. The cathode rays upon striking a zinc sulfide (fluorescent) screen, cause it to emit random flashes of light.
d. The cathode rays have a negative charge. This is shown by the fact that they are attracted toward positively charged metal plates held outside the tube.

3. Joseph John Thomson (1856-1940), an English physicist, after a series of investigations, established the particle nature of cathode rays.He determined the velocity these particles and the ratio of electric charge (e) to mass (m), e/m.

4. The value is about –1.759 x 108 coulombs/gram. He further concluded that cathode rays are negatively charged and are fundamental particles of matter. Later they were called electrons.

5. Thomson helped revolutionize the knowledge of atomic structure by his discovery of the electrons.

What is this sub particle?and what is its importance to our lives today? post your answer in my amazing forum by clicking here

To test if you have gained sufficient knowledge
test yourself click here a score of more than 70% is considered sufficient
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Dalton's Atomic Theory

Lesson Title: Dalton’s Atomic Theory

Learning Outcome:
At the end of the sessions, you must be able to:
1. Enumerate and explain the four assumptions made by John Dalton.
2. Explain the laws of chemical changes in terms of Dalton’s Atomic Theory.

Learning Presentation:

You have meet John Dalton previously, and as you have known he is the forerunner of today’s atomic theory, let us look closely his contributions click here

Another great references to have a full understanding of the contributions he made to the atomic theory is your textbook its on page 57 ;)

The four assumptions of John Dalton in his “Atomic Theory of Matter:
1. Each element is composed of tiny particles called atoms.

2. All atoms of a given element are identical; atoms of different elements are different and have different properties (e.g. masses).

3. Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created not destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

What do you think is the most important contribution of Dalton in the atomic theory formulation, is his theory still applicable today? How?
Send your answer by clicking here

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models of the atom

Lesson Title: Models of the Atom

Learning Outcome:
At the end of the sessions, you must be able to:
1. Know the different models of the atom

Learning Presentation:
Earlier I have let you imagine what is an atom, in this lesson we will see each of the different models as contributed by each of the scientist we have discuss earlier

JJ. Thomson Proposed a model of the atom click here to know more about it

Another scientist also studied the structure of the atom but it was more of an accidental discovery type he tried to verify the earlier atomic model but it resulted to a change in the idea of the structure of the atom click on the lick below to know more
Rutherford model of the atom click here to know more about it

Now from this model another scientist perfected the model of the atom and stipulated another atomic model, do you want to know what he found out? click below
Bohr Model click here to know more about it

To better grasp at the idea of the atom from its humble beginnings and to its present form today click on the link below
If you want to see the summary of the atomic model click here

Atomic models are also found in your textbook read it to have a fuller and better understanding of the model its on page 76 -77 :)

Did it match your initial idea of an atom? how?
visit my forum for this discussion click here

To test your knowledge try this game click here

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Development of the Atomic Theory

Lesson Title: Contributions of early Scientiest in the development of the Atomic Theory

Learning Outcome:
At the end of the sessions, you must be able to:
1. Trace the beginning of the development of the atomic theory.
2. Identify and recognize the significance of the contributions of early scientists in the development of atomic theory.

Learning Presentation
The atom is the most basic building block of matter, and chemistry is defined as the study of matter. Therefore to fully understand chemistry we must take a look at the atom.
What is an atom and what does it looks like?
I want you to imagine an atom and we will compare it later if your idea fits with the existing model of an atom.

Atoms though it may seem as a current idea was developed long ago. By Leucippus of Miletus and his student Democritus of Abdera.

Leucippus and Democritus: How did the two philosophers characterize the smallest unit of matter? What does the Greek word atomos mean?

Click on here to know how they came up with the idea of atoms

Another scientist gave evidence to support Democritus idea about atoms?
click here

John Dalton (1766-1844), an English chemist, integrated the results of the work of Boyle, Lavoisier and Proust and used them to provide the theory about the atomic nature of all matter. He was credited for the atomic theory. Why was the credit for atomic theory not given to Leucippus and Democritus when they were the first people to express the idea?

How significant were the studies of Boyle, Lavoisier and Proust in the development of atomic theory?

1. The idea of atoms as the smallest particle of matter started with Leucippus and Democritus about 400 BC.

2. In his study on gases, Robert Boyle (1627-1691) explained compressibility of air in terms of tiny particles forced closer together.

3. The Law of Conservation of Mass proposed by Antoine Lavoisier (1743-179) could be explained by atoms being indivisible. In chemical reactions, atoms only separate and/or unite with other atoms. No atoms can be destroyed nor created; thus, no change in weight of products is expected.

4. The Law of Definite Proportions or the Law of Definite Composition proposed by Joseph Louis Proust (1754-1826) could be explained using the idea of atoms. A compound is formed by the union of definite number ratio of combining atoms.

5. The Law of Multiple Proportions was put forward by John Dalton.

6. The three laws served to support the atomic theory proposed by Dalton in 1804. Without the work of these three scientists, Democritus idea of atoms could not have turned into a theory.

Visit my furom and give me a summary of the scientist that contributed to the atomic theory, also include the significance of wach discovery plus a summary of who do you think contributed the most to the atomic theory and why? click here to post answer

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